In remotely manned monitoring systems, the acoustic and/or visual signal generated by the sensor(s) is converted into an alarm message and generally transmitted through a radio link or on a telephone line, either public or private, to a receiving device that can be located far away from the monitored place at which the event detection occurred.
As an example, the alarm message can be sent either to a receiving device located in a central control unit manned by private personnel or by policemen, or to a fixed or mobile telephone set, furnished to the surveillance personnel or even to the owner of a house.
The alarm message received through said devices can incorporate information relating to the place at which the event took place, such as for example a predetermined recorded vocal message.
When the person in charge of the surveillance receives such alarm message from a monitored environment, he/she can either directly intervene or arrange for proper actions, such as for example the request for a police intervention.
One of the main inconveniences of the known alarm systems using volumetric sensors that transmit alarm messages to a remote control unit is that the surveillance personnel--or more generally the person that receives such alarm message--is not in condition to discriminate a false alarm caused by a disturbance from a real alarm situation in which a quick intervention is required.
The likelihood of a false alarm is not an infrequent event in spite of the technical improvements to the conventional antitheft systems.
When infrared sensors are used in a monitoring system, many disturbing sources such as natural or artificial light sources, quick changes of temperature, e.g. caused by room convectors, sudden raise of the environment lighting due for example to the front lights of a passing car, are all capable of being detected by the sensors and generating (false) alarm signals. Moreover, the sensitivity of infrared sensors decreases when the environment temperature increases.
When Doppler sensors are used, false alarms can be triggered by electromagnetic disturbances and by accidental movements of objects like a banging door or a falling flowerpot.
Because the alarm message received at the remote location does not contain information allowing the personnnel in charge to decide with certainty whether it is due to a real or a false alarm, such monitoring systems suffer from several drawbacks, such as delayed interventions, or unnecessary interventions, with a reduction of the system reliability.
For example, upon receiving an alarm message through his mobile phone, the owner of a dwelling that is far from home has to decide whether to ask for an intervention, or inform the police, or simply disregard the message as a false alarm, only on the basis of a generic alarm message he/she has received.
In the past several suggestions have been made in order to reduce the risk of false alarms, while attempting to still maintain the sensitivity of the sensors high.
One of the suggestions was to adjust the threshold level of the sensors to different conditions of the environment disturbances, another provided for pulse counters that actuated an alarm signal only after receiving a predetermined number of the sensor detections, more likely to be caused by the presence of an intruder moving within the protected enclosure.
Still another device provided for using a combination of Doppler and infrared sensors both of which had to be activated before an alarm message was generated.
However the above mentioned devices have not solved the problem of eliminating in the person receiving the alarm signal the uncertainty about its causes.
Namely it is known that alarm devices are not free from faults and malfuntions that can generate false alarms. Therefore, in the useful life of any alarm system a number of false alarms is to be considered as unavoidable.
Still with the aim of eliminating the uncertainity deriving from receiving an alarm signal, in the past there have been proposed devices that monitor the environment through a microphone or a television camera.
One of such devices is disclosed in FR-A-2 611 290 concerning an alarm device capable of transmitting an alarm message. The device illustrated and described in FR-A-2 611 290 comprises a volumetric sensor that upon being activated actuates a telephone dialing device which emits a predetermined stored number and transmits an alarm message through a telephone line.
At the end of the alarm message an environment hearing is started through a microphone that allows the remote listening of the sounds and noises within the monitored space.
However, the remote listening through microphones requires the installation of additional devices in the rooms to be monitored and does not allow the listening of low intensity noises, such as those caused by slow movements.
Moreover the background noise--due for example to the street traffic or to sound diffusing devices, such as radio and TV sets--is a strong source of disturbance to the remote listening through microphones, and can make impossible to distinguish between such noises and low intensity sounds caused by an intruder moving in the environment to be monitored.
A severe drawback to using microphones is set by law restrictions to environmental listening in force in some countries such as France, and anyhow such listening can be prolonged beyond given time limits.
The use of television cameras for visually monitoring at distance provides for a good perception of what is happening within the camera field of view, but at a very high cost, since at least one camera has to be installed in each of the rooms to be monitored, and the system further requires a receiving apparatus equipped with monitor(s) to display the images.